please help with time delay trigger

hello
could someone please help with that circuit?
#-Link-Snipped-#



I want to auto-trigger SCR (U1) when the capacitor is charged to a certain level
my understading is (I'm new at electronics, so sorry if that's very wrong 😀):
1) the SCR trigger needs a minimum current
2) the current on R2 wire is R=V/i
so supposing the minimum current to activate the SCR is 1mA and I want to trigger when capacitor is charged to 100V, I should use a R2=100M (I'm not using the real value in this schematics because I don't know the "i" yet)

does it make sense or I really need to use 555 (like in #-Link-Snipped-# ) or some other way?

(I couldn't simulate this on LTspice probably because I didn't get to specify the minimum current for SCR gate)

thanks